Breath testers of the type that employ an electronic detector are of relatively recent origin. These types of testers have many advantages over the older chemical type detectors. Included in these advantages are ease of portability and storage and ease of use. These advantages have led to this type of tester being employed in police work as a "screening" unit as described in U.S. Pat. No. 3,877,291, evidential units and in permanent installations such as coin-operated devices for use by the general public at, for example, drinking establishments.
Testers of this class are described in the following additional U.S. Pat. Nos. 3,764,270; 3,842,345; 3,886,786; 3,823,601; and 3,854,320, all of which are assigned to the same assignee as is the present invention.
In this type of tester it was thought to be necessary to read the peak of the detector output, which peak might occur after the exposure of the breath sample. A problem existed, however, in that the detector is sensitive to breath flow. That is, the output of the detector can vary if the subject continues to blow breath to the detector after the blowing period. Although the operator (e.g., a policeman) should remove the breath input unit and the tester from the subject at the conclusion of the flow, this is not always done or done soon enough, and the result is undesirable differences in outputs depending upon the operator.
One solution, suggested by U.S. Pat. No. 3,886,786 and others, is to provide a solenoid operated valve to stop the flow of breath to the detector at the conclusion of the predetermined flow (normally a time period). However, this approach is difficult and relatively expensive to implement in portable units and leads to problems in operation resulting from electric noise and air movement and vibration resulting from the operation of the valve.